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Astigmatism and the star test.

Started by Dave Sewell, 03/16/2013 08:46AM
Posted 03/16/2013 08:46AM Opening Post
Hello all - my optometrist has told me that I've got some astigmatism, but nothing serious enough to change my perscription for. My question is - would any astigmatism present in my own eyes possibly give me a false positive whenever I star test telescopes? I guess I could try rotating my head, while examining the out of focus diffraction rings to confirm the source. But first, I just wanted to know if it would even show up in the inside/outside of focus patterns. I realize that in focus, the stars might appear slightly cross-like in my case. I haven't had the chance to check this out yet through a telescope, but the other night, I did defocus my binoculars (of known excellent optical quality) while looking at Sirius, and the patterns did appear slightly oval, and flipped 90 degrees when going from inside of focus to outside of focus. Can someone with astigmatism out there clarify my question? Thanks so much - Dave
Posted 03/16/2013 09:59AM #1
Personal astigmatism would show up in the star test. But there are some caveats. Star tests are best done at high magnification, and eyeball astigmatism is less of an issue at such magnifications. The high mags use a smaller area of your eye that would eliminate some of the eye curvature issues.

Here is a good writeup by Mike Lockwood

http://www.loptics.com/articles/starshape/starshape.html